1. Field of the Invention
The subject matter of this invention is related generally to molded case circuit breakers and more specifically to trip units for molded case circuit breakers.
2. Description of the Prior Art
Molded case circuit breakers are well known in the art as exemplified by U.S. Pat. No. 5,910,760 issued Jun. 8, 1999 to Malingowski et al., entitled "Circuit Breaker with Double Rate Spring" and assigned to the assignee of the present application. The foregoing is incorporated herein by reference.
Molded case circuit breakers include a set of separable main contacts, one of which is usually fixed and one of which is movable for automatically opening upon the occurrence of an overload or short circuit electrical current in the network which the circuit breaker is provided to protect. The separable main contacts are opened as a result of the functioning of a latched operating mechanism, which is interconnectable by way of an operating handle to a region outside of the circuit breaker. The operating handle may be used to trip the circuit breaker manually or to reset and close the circuit breaker contacts once they have been opened automatically. The reset action is required because circuit breakers must be mechanically charged to be in a state to reopen immediately upon closure in the event that the fault which cause the tripping in the first place has not disappeared. The reset action charges the circuit breaker for that purpose. Molded case circuit breakers have trip units, which are often removably insertable in the circuit breaker case. The trip unit in addition has at least two calibratable functions, one of which is generally identified as thermal tripping and the other of which is generally identified as magnetic tripping. The trip unit includes a rotatable trip bar, which when rotated will actuate a latchable tripping operation within the operating mechanism to automatically open the circuit breaker contacts. The rotatable trip bar is usually actuated in one of two ways. The first way is in response to what is called a magnetic tripping of the circuit breaker. This occurs when the amount of current flowing through the separable main contacts of the circuit breaker is so high as to represent a potential catastrophic failure and which therefore requires exceedingly quick opening action of the circuit breaker. In such a case a electro magnetic core, which produces magnetic flux in proportion to the amount of electrical current flowing through the separable main contacts attracts a movable armature, the movement of which eventually causes the trip bar to move to thus cause the tripping action. The second tripping occurrence is in response to a relatively low amount of overload current, which eventually will cause overheating of the electrical wires in the circuit to be protected, but which does not necessitate the instantaneous action a short circuit requires and thus does not require the magnetic action spoken of previously. In this case a bi-metal element is heated by a heater element which conducts the electrical current flowing through the separable main contacts. As the bi-metal element flexes or moves it impinges upon the tripping bar causing it to flex and move correspondingly, until eventually a point is reached in which the tripping bar causes the circuit breaker to unlatch and trip automatically. Both the magnetic trip mechanism and the thermal trip mechanism usually require initial calibration.
In one half of an AC cycle, the electrical current flows through the circuit interrupter from the load by way of a terminal collar to the load terminal of the circuit breaker and from there into the trip unit where it flows through the previously mentioned heater which in turn is serially connected to the electro magnetic member of the magnetic trip device. From there it is interconnected by way of a flexible cable to one end of a moveable contact arm and from there to the main contact on the moveable contact arm. When the contact arm is closed, it is closed upon a fixed contact which is supported usually on u-shaped conductor, which in turn is interconnected with a line terminal and there to the line terminal collar and finally to the electrical line. In addition the circuit breaker usually has an arc chute for assisting in diminishing the electrical arc drawn between the separating contacts during the opening operation for extinguishing of the arc. The circuit breaker also has a slot motor arrangement, which is utilized to interact magnetically with the electrical current flowing in the opening contact arm to accelerate the opening of the contact arm magnetically. The operating mechanism usually consists of a series of levers and linkages, which are interconnected with the separable main moveable contact arm, the handle mechanism, and by way of a latch arrangement with the aforementioned trip bar. Description and operation of all of the above may be found in the previous mentioned, incorporated by reference '760 patent.
As was mentioned previously, occasions arise in which a reset of the circuit breaker is required. This implies that the circuit breaker must be latched in the open state until a positive action is taken by a operator to reset the circuit breaker, such as by actuating the handle mechanism to a reset position. In addition, it is necessary to latch the circuit breaker even in the closed state. The reason for this is that the amount of electrical current, which may give rise to an eventual opening of the circuit breaker, runs through a continuous range, whereas the circuit breaker when tripped must trip as quickly as possible. Thus an open contact latching arrangement is also required. In previous circuit breakers, trip units were an integral part of the circuit breaker case or were integrated into the circuit breaker case during the manufacturing operation. The latching for the circuit breaker was assigned to the operating mechanism region of the circuit breaker. Examples of such latching arrangements may be found in U.S. Pat. No. 4,594,491 issued Jun. 10, 1996 to Leone et al., and entitled "Molded Case Circuit Breaker with a Trip Mechanism Having an Intermediate Latch Lever" and assigned to the assignee of the present application. Another example may be found in U.S. Pat. No. 5,909,161. In the past the latching arrangements have called for and require the existence of latching components between the circuit breaker frame and the trip unit. This requires a difficult marriage between the frame and the trip unit. It would be advantageous if a primary latching arrangement could be provided in the trip unit in which a easy, simple circuit tripping operation could be provided between the trip unit and the operating mechanism or the main frame of the circuit breaker.